Non-volatile memory devices are currently in widespread use in electronic components that require the retention of information when electrical power is terminated. Non-volatile memory devices include read-only-memory (ROM), programmable-read-only memory (PROM), erasable-programmable-read-only memory (EPROM), and electrically-erasable-programmable-read-only-memory (EEPROM) devices. EEPROM devices differ from other non-volatile memory devices in that they can be electrically programmed and erased. Flash EEPROM devices are similar to EEPROM devices in that memory cells can be programmed and erased electrically. In Flash EEPROM devices, a group or block of memory cells may be programmed and/or erase together.
Product development efforts in EEPROM device technology have focused on increasing the programming speed, lowering programming and reading voltages, increasing data retention time, reducing cell erasure times and reducing cell dimensions. One conventional structure used for fabricating an EEPROM device is an oxide-nitride-oxide (ONO) structure. One EEPROM device that utilizes the ONO structure is a silicon-oxide-nitride-oxide-silicon (SONOS) type device. In a SONOS type device, an ONO stack is formed on a silicon substrate. A silicon control gate is then formed over the ONO stack.
In SONOS devices, during programming, electrical charge is transferred from the substrate to the silicon nitride layer in the ONO structure. Voltages are applied to the gate and drain creating vertical and lateral electric fields, which accelerate the electrons along the length of the channel. As the electrons move along the channel, some of them gain sufficient energy to jump over the potential barrier of the bottom silicon oxide layer and become trapped in the silicon nitride layer. Electrons are trapped near the drain region because the electric fields are the strongest near the drain. Reversing the potentials applied to the source and drain will cause electrons to travel along the channel in the opposite direction and be injected into the silicon nitride layer near the source region. Because silicon nitride is not electrically conductive, the charge introduced into the silicon nitride layer tends to remain localized. Accordingly, depending upon the application of voltage potentials, electrical charge can be stored in discrete regions within a single continuous silicon nitride layer.